Just check ETH and BTC graphs - on December 16 2017, Bitcoins prices was almost as today, but on the same day ETH price was around 700 USD. Right now Ethereum still falls behind Bitcoin. And on the time when Bitcoin reaches ATH, Ethereum will still be in few steps to ATH.
That is why agree what trauchot said, that the Bitcoins ATH price, Ethereum price will be around 600 USD.